SOLID-STATE ELECTROLYTE STUDY

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As an innovator in advanced lithium-ion battery technology, Enovix follows the work of others in the field closely. Researchers have long tried to develop safe, high-energy batteries using lithium (Li) metal as the active anode material. A July 12, 2017 article by David L. Chandler in MIT News, “Study suggests route to improving rechargeable lithium batteries,” reports on a study that indicates smooth surfaces may prevent harmful deposits from working their way into a solid electrolyte in a rechargeable Li-metal battery. The article opens as follows:

Most of today’s lithium-ion batteries, which power everything from cars to phones, use a liquid as the electrolyte between two electrodes. Using a solid electrolyte instead could offer major advantages for both safety and energy storage capacity, but attempts to do this have faced unexpected challenges.

Researchers now report that the problem may be an incorrect interpretation of how such batteries fail. The new findings, which could open new avenues for developing lithium batteries with solid electrolytes, are reported in the journal Advanced Energy Materials, [link added] in a paper by Yet-Ming Chiang, the Kyocera Professor of Ceramics at MIT; W. Craig Carter, the POSCO Professor of Materials Science and Engineering at MIT; and eight others.

The article describes several key findings from the study. First, contrary to a long-standing belief, the shear modulus of the solid electrolyte alone doesn’t eliminate lithium dendrites and resulting short circuits.

The problem, according to this study, is that researchers have been focusing on the wrong properties in their search for a solid electrolyte material. The prevailing idea was that the material’s firmness or squishiness (a property called shear modulus) determined whether dendrites could penetrate into the electrolyte. But the new analysis showed that it’s the smoothness of the surface that matters most.

Second, dendrites form differently in a solid electrolyte than they do in a liquid one.

In the last few years, a number of groups have been trying to develop solid electrolytes as a way of enabling the use of lithium metal electrodes. There are two main types being worked on, Chiang says: lithium phosphorus sulfides, and metal oxides. With all these research efforts, one of the prevailing thoughts was that the material needed to be stiff, not elastic. But these materials have tended to show inconsistent and confusing results in lab tests.

The idea made sense, Chiang says—a stiffer material should be more resistant to something trying to press into its surface. But the new work, in which the team tested samples of four different varieties of potential solid electrolyte materials and observed the details of how they performed during charging and discharging cycles, showed that the way dendrites actually form in stiff solid materials follows a completely different process than those that form in liquid electrolytes.

Third, it is the smoothness of the solid electrolyte’s surface that has the greatest effect on dendrite formation and propagation.

On the solid surfaces, lithium from one of the electrodes begins to be deposited, through an electrochemical reaction, onto any tiny defect that exists on the electrolyte’s surface, including tiny pits, cracks, and scratches. Once the initial deposit forms on such a defect, it continues to build—and, surprisingly, the buildup extends from the dendrite’s tip, not from its base, as it forces its way into the solid, acting like a wedge as it goes and opening an ever-wider crack.

These materials are “very sensitive to the number and size of surface defects, not to the bulk properties” of the material, Chiang says. “It’s the crack propagation that leads to failure. … It tells us that what we should be focusing on more is the quality of the surfaces, on how smooth and defect-free we can make these solid electrolyte films.”

Fourth, if the research is correct, practical application in a solid-state lithium battery will require minimizing structural defects at the lithium metal and electrolyte interface.

“I believe that this high-quality and novel work will reset the thinking about how to engineer practical lithium metal solid-state batteries,” says Alan Luntz, a consulting professor for metal-air battery research at Stanford University, who was not involved in this research. “The authors have shown that a different mechanism governs lithium metal shorting in lithium solid-state batteries than in liquid or polymer lithium metal batteries where dendrites form… This implies that if lithium metal solid-state batteries are ever to have practical current densities, then careful minimization of all structural defects at the lithium metal and electrolyte interface is essential,” he says.

The implications of the research are important. If correct, smooth surfaces will be needed to enable safe Li-metal batteries with solid state electrolytes. And this has the potential to significantly improve the energy density of rechargeable batteries. That being said, these R&D results are not a eureka moment, but rather a reset of the technology. The challenge now is to ascertain if and how smooth surfaces with very few and very minor defects can be practically and cost-effectively produced. This will determine whether Li-metal batteries with solid state electrolytes can be successfully commercialized.

Enovix has a decade of experience advancing a lithium-ion battery with increased energy density and improved safety from proof-of-concept research through development to pilot production. Through this experience, we’ve found two things to be true.

First, improvement in a single performance characteristic alone cannot significantly increase energy density and improve battery safety. Only improving multiple aspects of materials, design, and production can collectively accomplishment both.

Second, many promising technical discoveries made in the laboratory do not successfully transition to commercialization. For example, if doubling the energy density of a battery increases the cost 1,000%, it will not be successful in the marketplace. Technology discoveries that are too complex or too costly to produce cannot achieve successful commercialization.

One thing is certain. From wearables to automobiles, all mobile devices require better battery performance and safety if we are to achieve the promise of ¬an extraordinary mobile future. At Enovix, we are working to commercialize our 3D Silicon™ Lithium-ion Rechargeable Battery, with liquid electrolyte, for this reason. And we are rooting for other approaches, because it most likely will take a range of advanced battery technologies to address all the various applications and requirements.